1. Field of the Invention
The present invention relates generally to electro-explosive devices such as squibs and, more particularly, to such devices including protective means for preventing accidental ignition of the devices resulting from the presence of the device in an electromagnetic environment.
2. Description of the Prior Art
The electro-explosive device or squib is a fairly common detonator used to ignite an ordnance device such as a rocket, bomb, mine or other explosive charge into which the squib has been placed. The squib typically consists of a casing containing a heat-sensitive explosive material which is ignited by a bridge wire when the bridge wire is electrically heated by application of the electric current to the terminal wires of the squib. The bridge wire and the heat-sensitive explosive material are sealed within the casing in a waterproof manner with a packing material such as plastic, the terminal wires extending through the packing material out of the squib.
In a typical ordnance or explosive application the squib is embedded into a solid rocket propellant or explosive charge, with the terminal wires from the squib leading to a battery and triggering circuit. It is thus apparent that the wires between the squib and the battery triggering circuit may be anywhere from several inches to a number of feet in length.
Use of the squib device in an electromagnetic environment is a common occurrence, given the application of the squib device as a detonator for military ordnance. Such an electromagnetic environment may be caused by electromagnetic energy emanating from radar transmitters, telemetering systems, or high frequency communication equipment. When an electro-explosive device such as a squib with only a few inches of wire extending from the squib is located in such an electromagnetic environment, premature and unintended initiation of squib detonation may occur. Accordingly, protection of such squib devices from detonation due to an electromagnetic environment is essential.
Thus, the first requirement of the present invention is that the squib must be made completely immune from a surrounding electromagnetic environment. Of course, it may also be appreciated that the squib must have excellent reliability characteristics as well as an acceptable extended shelf life for use in a military application. In addition, the cost of providing a squib device with protection against premature detonation in an electromagnetic environment is an important consideration in terms of cost per unit. Since there are literally hundreds of thousands of squibs which have already been constructed, it is also desirable that the present invention be adaptable for use on an existing squib charge to prevent the immediate obsolescence of such existing devices.
Another important design requirement, and one nearly as critical as that of immunity to electromagnetic environments, is that the protected squib device have extremely low mass, particularly those portions of the squib which are metallic. This requirement is a result of the increasing use of squib charges to detonate solid propellent rocket motors used in shoulder-fired anti-armor assault weapons. Such weapons, which are descendants of the bazooka, are typically intended for a single use, after which the weapon is thrown away.
In such a device, the principal safety hazard is that of debris ejected from the device when it is fired. Such debris is principally the remnants of the squib device installed at the rear of the solid propellant used to fire the projectile, these remnants of the squib being discharged from the exhaust end of the weapon at high velocity. Thus, it can be seen that in order to minimize the potential of injury to personnel standing behind the firing position, the mass of the squib, particularly that of metallic portions contained in the squib, should be kept to an absolute minimum.
A further consideration as far as immunity to electromagnetic environments is concerned is that, typically for a military application, there must be a substantial built-in safety factor requiring that actual performance of the device far exceed the worst case condition which may be encountered. For the present application of the electro-explosive device or squib, the military standard typically requires that the maximum current induced in the bridgewire may not exceed 31.6% of the maximum no-fire current rating of the squib. It may therefore be appreciated that the standard imposed is fairly difficult to meet.
While it may be apparent that the subject prior art includes references dating back many years, upon examination it may swiftly be appreciated that these references are inapplicable to the present application. Early efforts in the field were focused mainly at safeguarding against electrostatic discharges, an earlier problem resolved by using coherer action as in U.S. Pat. No. 2,408,125, or by using means for producing a discharge at a location removed from the explosive material such as the discharge teeth taught in U.S. Pat. No. 2,408,125 or the spark gap of U.S. Pat. No. 4,261,263. Such devices are not pertinent to the features and objectives of the present invention.
A second approach is that of using a shunt capacitor as taught in U.S. Pat. Nos. 2,818,020 and 3,793,954. Other types of device include the SCR device of U.S. Pat. No. 3,640,224, which involves a time delay required to fire the squib, and the attenuator plug of U.S. Pat. No. 3,572,247. These approaches also do not deal with the particular novel aspects of the present invention, such as are described below.
Thus, it can be seen that the subject prior art does not include any devices having both the virtue of total immunity to high energy electromagnetic environments and the virtue of low mass to minimize ejected debris. While it would seem that such objectives seem mutually unachievable, it may also be appreciated that without both virtues construction of the type of ordnance contemplated by the present invention would be unachievable. Therefore, it can be seen that a substantial need exists for a squib device having a high immunity to high energy electromagnetic environments, low mass to minimize ejected debris, and good reliability characteristics and an extended shelf life, as well as a minimum cost to keep procurement expenses as low as possible. In addition, it is also desirable that the solution be achievable using existing squib devices to prevent making the hundreds of thousands of such devices existing prematurely obsolete.